1. Field of the Invention
The present invention relates generally to heating, ventilating, and air conditioning systems and more particularly to a controller of the type used to control ventilation within a laboratory.
2. Description of the Background of the Invention
Laboratory environments are typically provided with various types of ventilation equipment. One common piece of ventilation equipment is a fume hood. A fume hood is an enclosure having a movable door or doors, sometimes called a sash, at a front opening to provide access to the interior of the enclosure for conducting experiments, mixing chemicals, or the like. The fume hood is typically connected to an exhaust system for generating a flow of air through the fume hood to remove any noxious fumes or the like from the interior of the enclosure. Exhaust systems may include blowers that are capable of being driven at variable speeds to increase or decrease the flow of air from the fume hood to compensate for various sash positions. Alternatively, there may be single blower connected to an exhaust manifold that is in turn connected to the individual ducts exiting the fume hoods. Dampers may be provided in the individual ducts to modulate the flow from the individual ducts as required.
The velocity of the air flowing through the hood opening is called the face velocity. The average face velocity is typically defined as the flow of air into the fume hood per square foot of open face area of the fume hood, with the size of the open face area being dependent upon the position of the sash, and, in most types of enclosures, the amount of bypass opening that is provided when the sash is closed. The minimum acceptable face velocity is determined by the level of hazard of the materials being handled, and guidelines have been established relating face velocity to toxicity. Typical minimum face velocities for laboratory fume hoods are 75-150 feet per minute (FPM). The more hazardous the material being handled, the higher the recommended face velocity. However, too high a face velocity may cause turbulence, which can result in contaminants escaping from the hood. Additionally, high face velocities can be annoying to the operator and may damage fragile apparatus in the hood. As the sash position is varied, and the face velocity is changed due to changes in the toxicity of the materials used in the hood, it becomes increasingly difficult to maintain the face velocity constant.
To maintain a more constant face velocity as the sash is moved up and down, "bypass" hoods have been developed. A bypass hood has an opening, called a bypass opening, through which air can enter the fume hood when the sash is fully closed. Conversely, the bypass opening is blocked when the sash is fully opened. As the sash is moved from the fully closed to the fully opened position, the bypass opening is gradually moved from the fully opened to the fully closed position. Although bypass openings can provide some measure of control with respect to the face velocity, it is typically not possible to provide a bypass opening of the same size as the opening in the fume hood. Accordingly, it becomes necessary to vary either the blower speed or the position of the damper, depending upon the type of exhaust system in use.
It is known that the face velocity can be calculated by monitoring exhaust duct air velocity by means of a flow sensor. It has proved difficult, however, to provide sensors which reliably monitor air flow. Air flow sensors are costly and non-linear. They are also subject to contamination by materials in the exhaust air which lead to errors in calculating the actual face velocity. Attempts to use pressure sensors to measure flow have not met with success due to the very low pressure drops which typically exist in the exhaust duct.
To date, there are two primary methods employed in controlling fume hood face velocity. The most popular method uses a thermal anemometer, which indirectly measures face velocity through the hood side wall. This method provides direct measurement of face velocity, but exhibits slower than desired response characteristics. Further, the face velocity measurement can be affected by sensor position or hood setup. The second method controls the exhaust fan speed or positions a linear damper from a direct measurement of the sash position. That method has the advantage of great speed, but has the disadvantage of being unable to directly measure hood air flow or face velocity. That could result in large errors in face velocity over time, and is time consuming to set up.
U.S. Pat. No. 4,706,553 discloses a fume hood controller in which the sash position is monitored by a transducer that provides a signal indicative of the area of the hood opening. The variable speed motor controller is responsive to the sash position signal to provide a fan speed which varies in a substantially continuous, linear manner as a function of the sash opening. U.S. Pat. No. 5,090,303, is another laboratory fume hood controller which relies upon input signals indicating the sash position. The apparatus disclosed therein detects the position of each movable sash, calculates the size of the opening, measures the actual flow of air through the exhaust duct, and varies the flow of air through the exhaust duct in response to the position of the sash and the measured air flow.
There are a number of competing objectives in developing an alternative method for controlling fume hoods: decreased control response time; direct measurement of face velocity; simple installation, calibration and setup; simple control algorithm; and low cost. Problems encountered in trying to satisfy those objectives stem from the fact that the sash position may be changed very little, to provide a slight adjustment during an experiment, or a very large amount, very rapidly. Control systems set up to closely monitor minute changes in sash position typically take a long time to respond to large changes in sash position, particularly those changes which occur quickly. On the other hand, systems set up to quickly respond to large, fast, changes in sash position may be insensitive to small changes. Tradeoffs inevitably occur between response time and sensitivity. Thus, the need exists for a control system which is capable of responding quickly to changes in sash position without any loss in sensitivity.